1. Field of the Invention
The present invention relates to an imaging lens system and, particularly, to an imaging system of two-lens structure which is capable of reducing the size and weight and improving the optical performance and productivity. The imaging system is used for an image pickup device that forms images of objects such as scenery and human figures on an image taking surface of a solid image sensor element such as a CCD, CMOS, or the like, which is mounted on a portable computer, a television telephone, a cellular phone, and the like.
2. Description of the Related Art
Recently, there has been a remarkably increasing demand for a camera which utilizes a solid image sensor element such as a CCD, CMOS, or the like, which is to be mounted on a portable telephone, a portable computer, and a television telephone, for example. Such camera is necessary to be mounted on a limited mount space so that it is desired to be small and light.
Accordingly, an imaging lens system used for such camera is also required to be small and light. Thus, conventionally, a single-structure lens system using a single lens is used for such imaging lens system.
Such lens system with single-lens structure is sufficient when applied to a solid image sensor element referred to as CIF which has resolution of about 110,000 pixels. However, recently, use of a solid image sensor element referred to as VGA, which has higher resolution of about 300,000 pixels, has been investigated. In order to fully use the ability of the resolution of such solid image sensor element with the high resolution, the conventional lens system with the single-lens system is not sufficient.
Thus, conventionally, various types of lens systems with two-lens structure and three-lens structure have been proposed, which are more excellent in optical performance compared to that of the lens system with a single-lens structure.
In those cases, the three-lens-structure lens system enables to effectively correct each aberration that causes deterioration of the optical performance, thereby achieving extremely high optical performance. However, there are a large number of components required in the three-lens-structure lens system so that reduction of the size and weight is difficult, and manufacturing cost increases since high precision is required in each component.
On the contrary, the two-lens-structure lens system exhibits higher optical performance than the single-lens-structure lens system although it is not as high as that of the three-lens-structure lens system. Thus, it is considered a preferable lens system for a small and high-resolution solid image sensor element.
Conventionally, as such two-lens-structure lens system, a large number of so-called retro-focus type lens systems in which a negative lens and a positive lens are combined have been proposed. However, although such retro-focus type lens system can reduce the cost by decreasing the number of components, reduction of the size and weight close to that of the single-lens-structure lens system is practically not possible due to its structure where the back focus distance becomes long.
Further, as another two-lens-structure lens system, there is so-called a telephoto-type lens system in which a positive lens and a negative lens are combined. However, such telephoto-type lens system is originally developed for film photos so that the back focus distance is too short. Also, there is an issue of telecentricity so that it is difficult to use it as it is for an imaging lens system for a solid image sensor element.
Further, conventionally, the main stream structure of the two-lens-structure lens system or the three-lens-structure lens system has been formed to have a diaphragm disposed between two lenses which are adjacent to each other in the optical axis direction (for example, see Japanese Patent Unexamined Publication No. 2004-163850 and Japanese Patent Unexamined Publication No. 170460).
However, recently, there has been more increasing demand for still further improvement of the optical performance in this type of imaging lens system in addition to the reduction of size and weight. Thus, with the structure in which a diaphragm is disposed between two lenses as the imaging lens system disclosed in Japanese Patent Unexamined Publication No. 2004-163850 and Japanese Patent Unexamined Publication No. 170460, it is difficult to achieve both the reduction of size and weight and further improvement of the optical performance. Furthermore, it is difficult to align with the characteristic of a sensor (incident angle for the sensor).